In recent years, an apparatus conforming to an MPEG (Moving Picture Experts Group) scheme and the like is becoming widespread in both broadcast stations which distribute information and ordinary households which receive information, where the MPEG scheme treats image information as digital and compresses the information, for the purpose of efficiently transmitting and storing information, by means of an orthogonal transform such as a discrete cosine transform and motion compensation while exploiting redundancy specific to the image information.
MPEG-2 (ISO (International Organization for Standardization)/IEC (International Electrotechnical Commission) 13818-2), in particular, is defined as a general-purpose image encoding scheme that is a standard covering both an interlaced scan image and a progressive scan image as well as images with a standard resolution and a high resolution. The MPEG-2 is now used widely in a wide range of applications for use by professionals and consumers. The use of the MPEG-2 compression scheme can realize a high compression ratio and satisfactory image quality by assigning a code amount (bit rate) of 4 to 8 Mbps to an interlaced scan image with a standard resolution having 720×480 pixels, or 18 to 22 Mbps to an interlaced scan image with a high resolution having 1920×1088 pixels, for example.
The MPEG-2 has targeted high image-quality encoding adapted mainly for broadcasting, but has not been suited for an encoding scheme with a code amount (bit rate) lower than that of MPEG-1, namely, an encoding scheme with a higher compression ratio. The need for such encoding scheme is expected to increase in the future as a mobile terminal is coming into wide use. In response, an MPEG-4 encoding scheme has been standardized. With regards to the image encoding scheme, the standard was approved as an international standard ISO/IEC 14496-2 in December, 1998.
Furthermore, a standard named H.26L (ITU-T (International Telecommunication Union Telecommunication Standardization Sector) Q6/16 VCEG (Video Coding Expert Group)), which is initially intended for encoding an image used in a television conference, has been in the process of becoming standardized in recent years. While requiring a greater amount of calculation in encoding and decoding compared to the conventional encoding schemes such as the MPEG-2 and the MPEG-4, the H.26L is known to achieve higher encoding efficiency. As a part of the actions pertaining to the MPEG-4, moreover, a standardization achieving yet higher encoding efficiency by using the H.26L as a base and incorporating a function not supported in the H.26L has currently been in the process as Joint Model of Enhanced-Compression Video Coding.
The standard became an international standard in March, 2003 under the name of H.264 and MPEG-4 Part 10 (Advanced Video Coding; hereinafter noted as AVC) regarding the standardization schedule.
However, it was concerned that a macroblock size of 16 pixels×16 pixels was not optimal for a large picture frame provided in UHD (Ultra High Definition; 4000 pixels×2000 pixels) which is to be the target for a next-generation encoding scheme.
Accordingly, a JCTVC (Joint Collaboration Team-Video Coding) that is a joint standardization group formed by the ITU-T and the ISO/IEC is currently working to standardize an encoding scheme called an HEVC (High Efficiency Video Coding) for the purpose of further improving the encoding efficiency achieved by the AVC (refer to Non-Patent Document 1, for example).
The HEVC encoding scheme defines a coding unit (CU) as a processing unit similar to the macroblock used in the AVC. Unlike the macroblock used in the AVC, the size of the CU is not fixed to 16×16 pixels but specified within image compression information in each sequence.
Now, in order to improve encoding of a motion vector employing median prediction in the AVC, there has been proposed to adaptively use any of a “temporal predictor” and a “spatio-temporal predictor”, in addition to a “spatial predictor” required in the median prediction and defined in the AVC, as predictive motion vector information (hereinafter also referred to as MV competition) (refer to Non-Patent Document 2, for example).
In an image information encoding device, a cost function value when using respective predictive motion vector information for each block is calculated to select the optimal predictive motion vector information. Flag information indicating information on which of the predictive motion vector information is used for each block is transmitted to image compression information.
Moreover, there has been proposed a method called Motion Partition Merging (hereinafter also referred to as a merge mode) as one scheme of encoding motion information (refer to Non-Patent Document 3, for example). In this method, only the flag information is transmitted when the motion information for a current block is identical to the motion information for a neighboring block, and the motion information for the current block is reconstructed by using the motion information for the neighboring block at the time of decoding.
In order to parallelize a process, for example, the image encoding schemes such as the AVC and the HEVC provide a method in which a picture is divided into a plurality of slices so that a process is performed for each slice. An entropy slice is also proposed in addition to the slice.
The entropy slice is a processing unit adopted in an entropy encoding process and an entropy decoding process. That is, in the entropy encoding process and the entropy decoding process, a picture is divided into a plurality of entropy slices so that a process is performed for each entropy slice. Ina prediction process, on the other hand, the process is performed for each picture without applying the slice division.